For starters, as your link says, the analog trigger buttons don't work properly in DirectInput. Instead of registering as two independent axes, they show up as Left and Right on the same axis, so pressing both triggers at once looks the same as no-input. The triggers are generally the preferred control for primary buttons, so for all practical purposes XInput is required if you want to support the 360 pad.

(Their explanation is "the Xbox 360 controller was designed to register minimum value, not center, when the triggers are not being held." Seems like the obvious solution would be "don't do that.")